As for a dryer regeneration method in a so called closed air suspension system, there is disclosed a method for determining an amount of air required for regeneration based on an amount of suction air, a temperature or humidity of the suction air, in Patent document 1 as listed below, for example. Also, in Patent document 2 cited in the Patent document 1, there are disclosed various operation modes for supplying or discharging pressurized air between air chambers and a reservoir, in a vehicle level control.